In situ retorting of oil shale using nuclear energy



Sepp 19, 1967 R. B. JACOBS ETAL IN SITU RETORTINGOF -OIL SHALE USING NUCLEAR ENERGY Filed Dec.

T.. @s n@ \Js\s @u IN VEN T ORS Robe/ 5. Jacobs Lawrence Wright A TTOR/VEY N .mi

United States Patent O 3,342,257 IN SITU RETGRTING F OIL SHALE USING NUCLEAR ENERGY Robert B. Jacobs and Lawrence T. Wright, Homewood,

Ill., assigner-s to Standard Oil Company, Chicago, Ill.,

a corporation of Indiana Filed Dec. 30, 1963, Ser. No. 340,579 Claims. (Cl. 166--11) This is a continuation-in-part of application Serial No. 782,945, filed Dec. 24, 1958, now abandoned.

This invention relates to improvements in the recovery of oil from subsurface oil shale and similar formations. More particularly, the invention concerns the provision of a novel method for fracturing and in situ retorting of oil shale formation.

Despite the widespread occurrence of oil shale throughout much of the world, the large scale recovery of shale oil from such deposits has not been widely practiced. Barriers of geology, technology, and economics have heretofore effectively prevented more than token use of this source of oil. Geologically, many of the potentially most productive shales are covered by deep overburdens of earth and rock and, except in a few instances of outcroppings or surface valleys, are inaccessible for commercial recovery. Technologically, oil shale occurs as a relatively compact impermeable rock, which by present practice must be crushed or fractured by mechanical means before oil can be recovered by retorting the fragments; because of this impermeability, in situ retorting of oil shale has not met with success. From an economic standpoint, shale mining by open pit methods involves problems of overburden disposal, transportation to the refinery, crushing and grinding, and disposal of spent shale. Similarly, underground mining by gallery techniques and subsequent crushing and heating in special retorts is hardly suitable when considering current day liquid fuel requirements.

In accordance with the present invention, a method is provided for recovering oil from subsurface oil shale and similar formations which, in effect, constructs an underground retort in the formation itself and fills the retort with a supply of crumbled shale which is readily accessible to retorting gases. By this means, mining, transportation, and crushing and grinding problems which heretofore existed are obviated entirely. To employ the invention, an access well is drilled into the formation and a nuclear explosive device placed in the well near the bottom of the formation. Upon detonating the device, a large, substantially spherical hollow cavity is created instantaneously, the cavity being surrounded by a relatively thick Zone of fractured but impervious shale or rock. Immediately thereafter, fractured shale located directly above the cavity collapses and at least partially fills the cavity, resulting in a substantially cylindrical retort-like zone filled with crumbled and pervious shale. With many shales, a portion of the formation directly above the cylindrical cavity also collapses, and extends the cavity upward for a distance which may equal several times the height of the original spherical cavity; this shale also crumbles upon falling and hence also is pervious to gas fiow. Retorting the shale in the cylindrical retort thus formed is commenced by drilling input and recovery conduits to connect the cylinder with on-surface gas pumping and shale oil recovery facilities. A hot or heated gas, of a composition to be described more fully hereinafter, is then caused to flow through the crumbled pervious shale, preferably in a downward direction, thereby decomposing kerogen and distilling oil from the shale. This oil is collected and liows to the surface via the recovery conduits, as for example by pressurization or by pumping in well-known manner. After exhausting the oil shale fractured by the first ex- ICC plosion, an additional device may be detonated near the top of the first cylindrical zone and the recovery process repeated.

vThe invention will be more fully described with reference to the attached drawings wherein:

FIGURE l illustrates drilling of the access well and placing of the nuclear explosive device near the bottom of the oil shale formation.

FIGURE 2 shows the formation immediately after detonating the nuclear device.

FIGURE 3 depicts the promulgation of a cylindrical zone containing fractured and crumbled shale to be retorted.

FIGURE 4 shows the drilling of gas inlet and oil recovery conduits to connect with the previously established zone of fractured and crumbled shale, and illustrates a typical retorting operation.

FIGURE 5 shows the detonation of a second nuclear explosive device near the top of the cylinder produced by the first detonation.

FIGURE 6 is an alternate crushed shale.

The present invention utilizes the tremendous blasting effect of nuclear fission or fusion explosives to fracture the oil shale formation and thereby produce what is in effect an underground retort filled with crumbled pervious shale. To illustrate the formation 'and use of this retort-like system, attention -is invited to FIGURE l.

In FIGURE l, access well 1 is drilled from surface 2 through overburden 3 and into oil shale formation 4. The well is bottomed formation 4 in order to permit maximum utilization of the shale. The depth at which nuclear device 5 is planted depends in large measure on the depth of overburden 3 and shale formation 4; ordinarily, it is not desirable to detonate a nuclear device larger than about one kiloton (one kiloton is defined as the energy released by the explosion of 1,000 tons of tri-nitro toluene, tance of less than about 500 feet below surface 2, although this distance may be foreshortened if overburden 3 is composed of a well-consolidated rock. Suitable connections are then made to detonate nuclear device 5, and access well 1 is thereafter plugged, as for example by cementing the well according to well-known oil well plugging methods.

Upon detonating nuclear device 5, a sequence of events commences which is best understood by reference to FIG- URE 2. Within a few milliseconds after detonation, a substantially spherical bubble or cavity 6 is formed as a result of the tremendous heat and pressure exerted. This cavity is lined with molten rock or shale, in which substantially all of the radioactive fission products of the detonation will ultimately become imbedded. The molten lining is ordinarily only a few feet thick. Bubble 6 continues to expand `as a-result of radiation and mechanical shockwave pressures. As bubble 6 expands, the same pressures causing its expansion are transmitted to zone 7, a zone wherein the rock is forced into plastic flow and which has the configuration of an expanding concentric sphere. Shale or rock in zone 7 is not molten, but because of the high hydrostatic pressure and shear stresses contributed by the detonation of device 5 tends to assume conditions of plastic liow and actually deforms much as putty or similar plastic material 'does at lower shear stress. As strain proceeds through this material, the material becomes somewhat harder due to crystal strain hardening, and then begins to crack in both ductile and brittle fracture. Away from the material in plastic fiow in zone 7, the shale or rock in surrounding zone 8 exhibits elastic deformation, since its elastic limit has not been exceeded;

When detonating a 1.7 kiloton device underground, it is known that a spherical cavity or bubble 6 having a 55 foot method of retorting the preferably near the .bottom of shaleor 4X 109 B.t.u.) at a diS- 3 radius is formed, with zone 7 extending beyond cavity or bubble 6 an additional 75 feet. For a 20 kiloton device, 'the radius of bubble 6 is 125 feet, and for a 1000 kiloton (l megaton) device, the bubble is about 470 feet. It can be shown that, for all nuclear explosives, when bubble 6 is at its point of maximum expansion the pressure at its surface is about 40,000 pounds per square inch. In the case of a 1.7 kiloton explosive, the outward displacement of zone 7 caused by the formation of the cavity deiined by bubble 6 is 3.3 feet. This displacement occurs radially in all directions away from the original nuclear device The following table is presented to provide an indication of the tremendous amounts of useable shale which may be produced by the detonation of single nuclear devices of various energies. For the recovery of 50,000 barrels per calendar day (b./c.d) a shale (133 pounds per cubic foot) capable of yielding gallons of oil per ton of shale at 90% recovery must be retorted at the rate of 77,800 tons per calendar day. Bubble and cylinder volumes, together with the weight of broken shale and recoverable oil typically produced by nuclear explosions of the indicated magnitude are given below.

EXPLOSIVE PERFORLIANCE AND REQUIREMENTS FOR 50,000 B./C.D SHALE OIL and is compensated for by the compression of shale and rock in zone 8 of elastic ow.

Upon cooling, the pressure in cavity 6 diminishes to some low pressure, such as ground water pressure, and accordingly zone 7 which has been expanded plastically will be forced back toward the center of cavity 6 by stresses in elastic zone 8. However, these elastic stresses are considerably less than the pressure exerted by the explosion of device 5, and also the effect of strain hardening of the shale becomes important. Thus, a void space is created which heretofore did not exist. In the case of a 1.7 kiloton explosive, the inward radial displacement of bubble 6 after the explosion and expansion is about 6 feet, in comparison with the maximum initial displacement of 61 feet. Consequently, the new radius after elastic return is about 55 feet, with this volume representing a newly created void space. Similarly, the outer limits of zone 7 in a 1.7 kiloton explosion are displaced inwardly by about 1.1 ft.

Immediately after contraction of bubble 6, and perhaps even while the contraction is taking place, the portion of zone 7 which lies directly above bubble 6 commences to collapse and drop into the cavity of the bubble. This may be depicted with reference to FIGURE 3. It will be recalled that zone 7 consists of fractured and reconsolidated shale, and although this material is impervious to water or gas flow, it has extremely low tensile strength. As a result, it readily crumbles into the cavity produced by bubble 6, filling it with crumbled pervious shale 9. The volume of crumbled shale 9 (on an in-place basis) is approximately that of a cylinder having a radius equal to the diameter of bubble 6 and a height equal to the distance between the top of bubble 6 and the top limits of zone 7. This cylinder may taper inward somewhat toward the top. At the bottom of the cylinder, the upper half of fused bubble 6 lies in the form of glass-like fragments atop the relatively intact hemisphere that was formerly the lower half of bubble 6.

When the cylinder is formed in zone 7 by the collapse of the upper portion thereof, the roof directly above this becomes unsupported. Due to the low tensile strength of the shale roof, more or less of this roof will also collapse into zone 9, thereby adding to the inventory of retortable oil shale.

The above examples illustrate the minimum volume of crumbled shale, and oil recoverable therefrom, when the collapse of the shale roof extends only to the upper limit 0f Zone 7. Additional crumbled shale becomes available if the roof collapse extends above zone 7. The producible oil per detonation decreases linearly with size, and accordingly explosive devices having an energy of as little as 0.1 kiloton may be used.

After establishing cylinder 9 by detonation of nuclear device 5 and the resultant collapse of shale located above bubble 6, one or more gas inlet conduits and oil recovery conduits are connected to communicate with the upper and lower portions of cylinder 9. As shown in FIGURE 4, the inlet 10 is preferably drilled to connect with cylinder 9 at a point near the top thereof, while oil recovery conduit 11 may be drilled through the fractured and reconsolidated zone 7 (which, unlike the crumbled rubble-like shale in cylinder 9, is capable of providing lateral support for the drill stem) and then directed by well known means, e.g., whip-stocking, to connect with cylinder 9 immediately above the bottom of the lower hemispherical half of bubble 6. Thus, the intact lower half of bubble 6 acts as a trough to collect descending oil. As will appear hereinafter, gas inlet 10 may alternatively be connected into the lower portion of cylinder 9 while outlet 11 may communicate with the top. This latter arrangement permits somewhat more of the heat liberated by nuclear device 5 to be employed in retorting the oil shale.

Since the heat generated by the explosion of either iission or fusion devices is only sucient to retort at most about 10% of the recoverable oil, it is essential to provide heat either by an external or by an internal source to accomplish satisfactory retorting and oil recovery. Thus, a restorting gas is used to heat the formation, with the retorting gas itself being heated either above ground or below ground, as for example by burning air or other oxygen-containing gas with a combustible-containing gas such as methane or make gas. Depending upon the desired oil recovery and on the physical nature of the oil shale formation, i.e., its oil content, Whether it yields its oil readily, whether it crumbles into small readily-retorted fragments, etc., the optimum retorting gas composition may be selected from among many of such gases which are available. For example, a heated oxygen-free gas,

which may be heated at the surface and introduced hot into cylinder 9, is best employed for hard shales which do not shatter. These hard shales tend to form rather large rocks or boulders which require a retorting period on the order of several days to permit central portions of each boulder to become suliciently heated to liberate the oil. For these shales, retorting must be conducted slowly. Alternatively, the retorting gas may contain oxygen with the balance being either combustible or non-combustible, in whole or in part.

Preferred retorting practice constitutes the partial or complete combustion of carbonaceous material remaining in the oil shale formations after distillation of the oil. This procedure, termed in situ combustion, may be conducted in various ways. Briefly, it is commenced by establishing a hot zone in the formation of sufficient temperature to support combustion; this temperature decreases with pressure, but for atmospheric pressure is at least about 600 F. The hot zone may be established by such means as burning an oxygen-containing gas with a combustible gas, by electric heating, or by chemical heating using such incendiaries as thermit.

Once the hot zone is established, it is advanced in the desired direction by continuous or intermittent injection of a gas which advances the heat front. Advancement of the hot zone may be by (l) continuous injection of air or other non-combustible gas containing at least about 20 volume percent oxygen, (2) introducing a gas having low oxygen content, desirably between one and 20%, preferably between about one and six volume percent, the balance being either combustible or non-combustible, to form a sharp heat wave, (3) injection of an oxygen-containing gas for a short period, followed by halting combustion at a predetermined distance from the original heated zone and thereafter introducing only an essentially oxygen-free gas to abstract heat from the already-heated zone and employ this heat in retorting oil, and (4) various combz'nations of one or more of the above techniques, e.g., introducing air to generate a rather broad burning zone, and thereafter recycling oxygen-free gas, followed by air introduction whenever the temperature drops below a sufficient level which can effect retorting. The basic principle in the above methods involves the creation and establishment of a hot zone, and the passing of a gas through that zone to become heated and, in its further ad- Vance through the as-yet-unheated crumbled shale, provides heat to decompose the kerogen and retort the oil.

When employing air or other gas containing at least about 20% oxygen, the initially established hot zone is advanced through retort-like cylinder 9 as a broad heat wave. The maximum temperature and rate at which the heat wave is propagated depends upon the amount of residual combustibles in the formation, on the oxygen content of the introduced gas, and on the mass rate of flow of oxygen. Thus, with a constant oxygen-containing gas composition, a shale which -contains a relatively large amount of residual carbon will generate a higher temperature, the peak of which will advance at a slower rate than a shale which has less carbonaceous residue. Similarly, with the same shale, the peak combustion temperature may be increased by increasing the oxygen content of the burning gas, and the combustion rate may be increased by raising the gas input rate.

The introduction of air or other high oxygen content gas results in yielding a high oil recovery per unit volume of introduced gas. It may not, however, be economical in all situations, especially in those cases Where the crumbled shale becomes well packed and tends to form liquid blocks if vaporization proceeds faster than condensed oil can advance through cooler portions of the formation. In this event, it may be desirable to employ a gas of reduced oxygen content, below 20%, e.g., l-6%, with the balance being either nitrogen, recycled make gas, or engine exhaust gas. Once the hot zone is initially established 'by ignition or burning of gas near inlet 10,

gas is injected while relatively cool, e.g., 60-150 F., into the formation to advance the burning zone. In this type of operation, the heat wave has a sharp profile, c-aused -by cooling of the formation behind the burning zone by the cool gas. The peak temperature at the burning zone is somewhat lower than when a gas of higher oxygen content is employed. However, as a result of this lower peak temperature controlled burning is somewhat more easily attained. At the same time, however, a larger volume of gas relative to air injection must be employed, and to obtain economies in gas compression costs it is desirable to conduct the retorting operation under superatmospheric pressures, eg., 50-500 p.s.i.g. Pressure operation can be secured by maintaining back pressure on recovery conduit 11.

An alternative method, and one which is especially suitable for use with small, e.g., 10-100 kiloton, bombs is the advancement of the burning zone a limited distance into the crumbled shale in cylinder 9 by means of an oxygen-containing gas, and thereafter introducing a substantially oxygen-free gas to accomplish retorting of at least a portion of the remaining unburned shale by means of heat absorbed by the oxygen-free gas as it passes through the heated zone.

Another method of in situ combustion involves the alternation of an oxygen-containing gas with an oxygenfree gas. The former, which illustratively may be air, advances the combustion zone while the oxygen-free gas performs the major portion of the retorting by absorbing heat from the previously established combustion zone and advancing it into the crumbled shale to be retorted. The cycle is repeated as often as is necessary to maintain a combustion zone tempera-ture of at least about 700 F.

During retorting with either an oxygen-free gas or a gas produced lby in situ combustion of make-gas or remaining kerogen, the employment of pressure in cylinder 9 is of advantage. Pressures, advantageously on the order of 50-500 p.s.i.g., increase the rate of heat zone advance, afford a gas of higher specific heat per unit volume to facilitate distillation, and allow liquid oil to be pressured to the surface via oil recovery conduit 11. In addition, it reduces compression cost by permitting recycle of the retorting gas. Control of either a pressure of non-pressure retorting operation is achieved by regulating the volume, pressure, and temperature of injection gas and by manipulating back pressure on recovery conduit 11. Analysis of produced gas, and temperature measurements when available, are used to determine the progress and condition of the burning zone.

The necessary amount of retorting gas is a function of such variables as retorting gas temperature, ease of retorting the particular oil shale, extent of oil recovered, initial shale temperature, and several other factors. In general, however, with burning zone temperatures between about 700 and about 2500 F., a retorting gas rate on the order of 20 to about 800 standard cubic feet of retorting gas (Volume of gas advancing into the cool shale formation) per cubic foot of crumbled shale in cylindrical zone 9 affords satisfactory recovery.

Recovery of distilled oil through recovery conduit 11 may follow normal techniques such as are employed in the recovery of oil from oil sands. Thus, for example, if retorting is conducted under suflicient pressure it is possible to utilize a single line for conduit 11 and to pressure the mixture of retorting gas, make gas, and liquid or vaporized oil to the surface directly. In deep wells it may be desirable to utilize a -concentric tube and casing for conduit 11; in this event, the casing is perforated at a height above the opening of the concentric tube, While the tube itself extends to the bottom of fused bubble 6. Thus, gases and vapors are exhausted via the casing, while the tube extends into a liquid pool of rethe low-oxygen content torted oil which may then be pressurized or pumped to the surface.

An additional shale oil refining benefit accrues when conducting the retorting operation in a vertical downward direction. It was previously indicated that the oncemolten bubble contains virtually all of the radioactive fission products of the nuclear detonation, and as a result the bubble material is lhighly beta-'and gamma-active. Thus, when liquid shale oil, particularly at the relatively high retorting temperatures of '700 F. and. higher, lis collected in the trough, it is being bombarded by the nuclear radiation from the trough itself. This radiation promotes the radio-thermal cracking of shale oil to lighter boiling hydrocarbons.

In a `specific embodiment of our process, oil is retorted from a shale seam which is 450 feet in thickness and carries an overburden of 800 feet of rock. The shale seam contains an average of 30 gallons of oil per ton (Fischer assay) and has an in-place density of 133 pounds per cubic foot. A ten kiloton nuclear dev-ice is disposed in a well near the lower interface of the shale seam and the underlying rock. Thereafter, the well is sealed.

Upon detonation of the device, a bubble (zone 6) having a radius of 99 feet is formed. The fractured zone (zone 7) outside the bubble zone has a radius of 235 feet. After detonation of the device and subsequent caving-in of the shale which occupied the cylinder above the bubble, a roughly cylindrical mass of crumbled shale amounting to about 280,000 tons is formed having a depth of 225 feet, a diameter of 198 feet, and 40% voids.

The crumbled shale formation is then provided with inlet means for introducing hot retorting gas into the upper portion of the cavity and recovery means for recovery from the bottom of the mass of crumbled shale, and bringing to the earths surface, gases and liquids. A retorting gas comprising essentially nitrogen and being substantially free of molecular oxygen is heated to 1000 F. and int-roduced through the inlet means into the cavity at the rate of two mols per hour per square foot of crosssectional area of the cylindrical mass of crumbled s-hale. The inlet pressure of the retorting gas is suflicient, 2040 p.s.i.g., to maintain 'about a 5-l0 p.s.i.g. pressure at the surface outlet of the recovery means. Gases and ycondensed liquids are withdrawn and transported to the surface via the recovery means. Continuing the flow of hot retorting gas as aforesaid for 550-600 hours completes the heating of the shale (the bottom portion of the shale having reached a temperature of 750 F.) and the recovery at the surface of 179,000 barrels of liquid oil produced.

Turning now to FIGURE 5, a method is there shown for employing a plurality of nuclear devices either for recovering oil from very deep formations or for using nuclear devices of low energy. This method essentially consists of detonating an additional device near the top of cylinder 9 after substantially all of the shale therein has been retorted. In this aspect of the invention, a new access well 13 is drilled to connect near the top of former cylinder 9; alternately, gas inlet conduit 10 may be employed for this purpose. A nuclear explosive device is then placed near the bottom of access well 13 and the well sealed off as for example -by cementing. Gas inlet conduit and oil recovery conduit 11 may be sealed by either permanently or temporarily blocking the respective conduits. The device is then detonated, causing the formation of a new bubble 14 and `a new zone of fractured and reconsolidated shale 15, the upper portion of which collapses as in the previous explosion to form a cylindrical retort 16 containing fractured and crumbled shale which is pervious to gas flow. The lower hemispherical portion of bubble 14 remains substantially intact and serves as an oil-connection trough in the event retorting is conducted vertically downward. New conduits, or the former old conduits 10 and 11, are connected to cylinder 16; this is shown in the drawing as inlet conduit 10 and oil recovery conduit 11a. Retorting is then conducted as in the previous detonation.

FIGURE 6 shows an alternate method of retorting the crushed shale which is especially advantageous when retorting the shale produced by the detonation of large explosive devices or when conducting the retorting operation in an upward direction. This method essentially consists of raising the opening of oil recovery conduit 11 to correspond with the level at which the retorted oil is present substantially as a vapor or mist. Thus, as the lhot retorting gas releases oil vapor from the crumbled shale, conduit 11 is either gradually elevated or additional inlet ports are perforated therein to effectively raise the level at which oil is collected. This avoids condensing the oil as liquid on upper portions of cylinder 9. This method is suitable either for -preheated gas retorting or in situ combustion gas retorting.

The present process may lalso be employed with advantage in the recovery of oil from Athabaska tar sands and similar formations. Tar sands are found in the province of Alberta in Canada as well as in several other locations and consist of sand to which a highly viscous oil or tar adheres. This oil is apparently different from kerogen since, unlike the latter, it can be extracted from the formation by ordinary hydrocarbon solvents such as benzene or hexane. The oil is a heavy (5 API) hydrocarbon with a 60 F. pour point, a viscosity of about 3,000,000 centipoises at ambient temperature, diminishing sharply to one centipoise at 20D-250 F. Beds of tar sand frequently are -200 feet thick with a sand-clay overburden varying from 15 to 2000 or more feet deep. Limestone underlies the tar sand. Because of the relatively thin beds, fission or fusion devices of relatively small size, e.g., 0.1100 kiloton, detonated at the bottom or slightly below a thick section of the bed constitutes the preferred technique. The recovery procedure follows essentially the same pattern as in the recovery of shale oil.

From the foregoing presentation it is clear that the inventive process affords an ideal method for the low cost recovery of shale oil from oil shale formations. By detonating a nuclear explosive device in or near the formation, thereby creating a retort-like cylindrical cavity filled with crushed and pervious shale, and by flowing a retorting gas through the shale to decompose kerogen and release the shale oil, it is possible to recover oil from formations which have hitherto been considered too inaccessible or too expensive for conventional recovery techniques.

Obviously, many variations are possible within the scope of the contemplated inventive process. For example, although FIGURE 1 shows access well 1 in a vertical direction, the lower portion 1a of access well 1 may be drilled in a transverse or dogleg manner so as to eliminate the necessity of drilling gas inlet conduit 10 and oil recovery conduit 11 either as separate conduits or by re-drilling plugged access well 1. By using a dogleg access well, nuclear device 5a may be planted in an offset direct from the main access well 1. Consequently, only the lowest portion of access well 1, if at all, need be plugged to confine the nuclear explosion, since the use of an offset access well permits the explosion to be confined because the main access well is collapsed and sealed off before the escape of radioactive material. Since, in this case, access Well 1 need not be plugged prior to detonation of device 5a, access well 1 together with appropriate offset wells may be used to furnish the hole for gas inlet conduit 10 and/ or oil recovery conduit 11.

We claim:

1. An improved method of recovering shale oil from a subsurface oil shale formation with a nuclear explosive device, which comprises: drilling an access well into the formation, inserting a nuclear explosive device in said well near the bottom of said formation, sealing said well above said formation to confine the subsequent nuclear explosion, said device having an energy-yield relationship with the depth of placement suicient to insure containment of the subsequent explosion, detonating said device thereby to create a cavity in said formation, which cavity :at least partially lls with collapsing oil shale to form a zone of fractured and crumbled shale of high permeability, connecting at least one gas input conduit and at least one oil recovery conduit to said zone, flowing hot retorting gas through said gas input conduit into at least part of said fractured and crumbled shale in a generally vertical direction to heat the shale and distill the shale oil therefrom, and recovering the shale oil via the oil recovery conduit.

2. An improved method of recovering shale oil from a subsurface oil shale formation with a nuclear explosive device, which comprises: drilling an access well into the formation, inserting a nuclear explosive device in said well near the bottom of said formation, sealing said well above said formation to confine the subsequent nuclear explosion, said device having an energy-yield relationship with the depth of placement sufllcient to insure containment of the susbequent explosion, detonating said device thereby to create a spherical cavity in said formation, which cavity at least partially fills with collapsing oil shale to form a cylindrical zone of fractured and crumbled shale of high permeability, connecting a gas input conduit near the top of said cylindrical zone and an oil recovery conduit near the ybottom thereof, flowing hot retorting gas through said gas'input conduit into at least part of said fractured and crumbled shale in a generally vertical downward direction to heat the shale and distill the shale oil therefrom, and recovering the shale oil via the oil recovery conduit.

3. An improved method of recovering shale oil from a subsurface oil shale formation with a nuclear explosive device, which comprises: drilling an access well into the formation, inserting a nuclear explosive device in said Well near the bottom of said formation, sealing said well above said formation to confine the subsequent nuclear explosion, said device having an energy-yield relationship with the depth of placement suilicient to insure containment of the subsequent explosion, detonating said device thereby to create a spherical cavity in said formation, which cavity at least partially fills with collapsing oil shale to form a cylindrical zone of fractured and crumbled shale of high permeability, connecting a gas input conduit near the bottom of said cylindrical zone and an oil recovery conduit near the top of said zone, flowing hot retorting gas via said gas input conduit into at least part of said fractured and crumbled shale in a generally vertical upward direction to heat the shale and distill the shale oil therefrom, and progressively withdrawing the recovery conduit as retorting progresses to recover shale oil without excessive condensation on cool shale.

4. An improved method of recovering shale oil from a subsurface oil shale formation with a nuclear explosive device. which comprises: drilling an access well into the formation, inserting a nuclear explosive device in said well near the bottom of said formation, sealing said Well above said formation to confine the subsequent nuclear explosion, said device having an energy-yield relationship with the depth of placement suflicient to insure containment of the subsequent explosion, detonating said device thereby to create a substantially spherical cavity in said formation, which cavity at least partially lills with collapsing oil shale to form a substantially cylindrical zone of fractured and crumbled shale of high permeability, connecting at least one gas input conduit and at least one oil recovery conduit to said zone, flowing hot retorting gas through said gas input conduit into at least part of said fractured and crumbled shale in a generally Vertical direction to heat the shale and distill the shale oil therefrom, and recovering the shale oil via the oil recovery conduit.

5. An improved method of recovering shale oil from a subsurface -oil shale formation with a nuclear explosive device, which comprises: drilling an access Well into the formation, inserting a nuclear explosive device in said well near the bottom of said formation, sealing said well above said formation to confine the subsequent nuclear explosion, said device having an energ -yield relationship with the depth of placement sutlicient to insure containment of the subsequent explosion, detonating said device thereby to create a spherical cavity in said formation, which cavity at least partially fills with collapsing oil shale to form a cylindrical zone of fractured and crumbled shale of high permeability, connecting a gas input conduit and an oil recovery conduit to said zone, flowing hot retorting gas comprising the combustion products of air and a portion of the kerogen remaining with the oil shale after distillation of shale oil therefrom into at least part of said fractured and crumbled shale in a generally vertical direction to heat the shale and distill the shale oil therefrom, and recovering the shale oil via the oil recovery conduit.

6. An improved method of recovering shale oil from a subsurface oil shale formation with a nuclear explosive device, which comprises: drilling an access well into the formation, inserting a nuclear explosive device in said well near the bottom of said formation, sealing said well above said formation to confine the subsequent nuclear explosion, said device having an energy-yield relationship with the depth of placement sullicient to insure containment of the subsequent explosi-on, detonating said device thereby to create la spherical cavity in said formation, which cavity at least partially fills with collapsing oil shale to form a cylindrical zone of fractured and crumbled shale of high permeability, connecting a gas input conduit and an oil recovery conduit to said zone, flowing hot substantially oxygen-free retorting gas heated to retorting temperature prior to introduction into said cylindrical Zone to said Zone and passing said retorting gas through at least part of said fractured and crumbled shale in a generally vertical direction to heat the shale and distill the shale oil therefrom, and recovering the shale oil via the oil recovery conduit.

7. An improved method of recovering shale oil from a subsurface oil shale formation with a nuclear explosive device, which comprises: drilling an access well into the formation, inserting a nuclear explosive device in said well near the bottom of said formation, sealing said well above said formation to confine the subsequent nuclear explosion, said device having an energy-yield relationship with the depth of placement suflicient to insure containment of the subsequent explosion, detonating said device thereby to create a spherical cavity in said formation, which cavity at least partially iills with collapsing oil shale to form a cylindrical zone of fractured and crumbled shale of high permeability, connecting a gas input conduit and an oil recovery conduit to said zone, flowing hot retorting gas through said gas input conduit into at least part of said fractured and crumbled shale in a generally vertical direction to heat the shale and distill the shale oil therefrom, recovering the shale oil via the oil recovery conduit, placing at least one additional nuclear explosive device near the top of said cylindrical zone, detonating said at least one additional nuclear explosive device, flowing hot retorting gas through the freshly fractured and crumbled shale thus produced to heat said shale and distill the shale oil therefrom, and recovering the additional shale oil.

8. An improved method of recovering shale oil from a subsurface oil shale formation with a nuclear explosive device, which comprises: drilling an access well having an offset or dogleg portion extending into the formation, disposing a nuclear explosive device in said formation via said well, sealing said Well above said formation to confine the subsequent nuclear explosion, said device liaving an energy-yield relationship with the depth of placement of said device suicient to insure containment of the subsequent explosion, detonating the nuclear explosive device to create a spherical cavity in said formation, which cavity at least partially lls with collapsing oil shale to form a cylindrical zone of fractured and crumbled shale of high permeability, completing a gas input conduit via said access well near the top of said zone, completing a recovery conduit via said access well near the bottom of the original spherical cavity, flowing a hot retorting gas vertically downward through at least part of said fractured and crumbled shale, thereby distilling and separating the shale oil from said shale, and recovering the shale oil thus produced via said recovery conduit.

9. An improved method of recovering shale oil from a subsurface oil shale formation with a nuclear explosive device, which comprises: drilling an access well into the formation, disposing a nuclear explosive device in said formation via said well sealing said well above said formation to confine the subsequent nuclear explosion, said device having an energy-yield relationship with the depth of placement of said device sufficient to insure containment of the subsequent explosion, detonating the nuclear device to create a spherical cavity in said formation, which cavity at least partially lls with collapsing oil shale to form a cylindrical zone of fractured and crumbled shale of high permeability, connecting a gas input conduit near the top of said zone, connecting a recovery conduit near the bottom of the original spherical cavity,

owing a hot retorting gas vertically downward through atleast part of said fractured and crumbled shale, thereby distilling and separating the shale oil from said shale, and recovering the shale oil thus produced via said recovery conduit.

10. The method of claim 9 wherein said nuclear explosive device has an energy of between about 0.1 and about one hundred kilotons.

References Cited UNITED STATES PATENTS 1,422,204 7/1922 Hoover et al 166-11 2,481,051 9/ 1949 Uren 166-39 X 2,630,307 y3/1953 Martin 166-11 2,819,761 1/1958 Popham et al 166-11 X 2,825,408 3/1958 Watson 166-11 3,001,776 9/1961 Van Poollen 166-11 X 3,113,620 12/ 1963 Hemminger 166-36 OTHER REFERENCES Rougeron, Les Applications de lEXplosion Thermonucleare, Paris 1956, editions Berger-Leverault, 5 -rue Auguste Compte (VI) 1956, pp` 192-194.

The Washington Post and Times-Herald, November 12, 1958, p. B-12.

CHARLES E. OCONNELL, Primary Examiner.

STEPHEN J. NOVOSAD, Examiner.

UNITED STATES PATENT oFFICE CERTIFICATE 0E CORRECTION Patent No. 3,342,257 September 19, 1967 Robert B. Jacobs et al.

It is hereby certified that error appears in the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 4, in the table, the second heading under the heading "Nuclear Type", for "Fission" read Fusion Same table, first column, line 6 thereof, for "Recoverabel" read Recoverable Same column 4, line 65, for "restorting read retorting column 7, line 73, for "Connection" read collection column 9, line 57, for "device." read device,

Signed and sealed this 29th day of October 1968.

(SEAL) Attest:

EDWARD J. BRENNER Commissioner of Patents Edward M. Fletcher, Jr.

Attesting Ufficer 

1. AN IMPROVED METHOD OF RECOVERING SHALE OIL FROM A SUBSURFACE OIL SHALE FORMATION WITH A NUCLEAR EXPLOSIVE DEVICE, WHICH COMPRISES: DRILLING AN ACESS WELL INTO THE FORMATION, INSERTING A NUCLEAR EXPLOSIVE DEVICE IN SAID WELL NEAR THE BOTTOM OF SAID FORMATION, SEALING SAID WELL ABOVE SAID FORMATION TO CONFINE THE SUBSEQUENT NUCLEAR EXPLOSION, SAID DEVICE HAVING AT ENERGY-YIELD RELATIONSHIP WITH THE DEPTH OF PLACEMENT SUFFICIENT TO INSURE CONTAINMENT OF THE SUBSEQUENT EXPLOSION, DETONATING SAID DEVICE THEREBY TO CREATE A CAVITY IN SAID FORMATION, WHICH CAVITY AT LEAST PARTIALLY FILLS WITH COLLAPSING OIL SHALE TO FORM A ZONE OF FRACTURED AND CRUMBLED SHALE OF HIGH PERMEABILITY, CONNECTING AT LEAST ONE GAS INPUT CONDUIT AND AT LEAST ONE OIL RECOVERY CONDUIT TO SAID ZONE, FLOWING HOT RETORTING GAS THROUGH SAID GAS INPUT CONDUIT INTO AT LEAST PART OF SAID FRACTURED AND CRUMBLED SHALE IN A GENERALLY VERTICAL DIRECTION TO HEAT THE SHALE AND DISTILL THE SHALE OIL THEREFROM, AND RECOVERING THE SHALE OIL VIA THE OIL RECOVERY CONDUIT. 